Prior to the present invention, cyclopolydimethylsiloxanes were prepared by hydrolyzing dimethyldichlorosilane in the presence of an effective amount of cationic surface active agent such as a protonated amine or a quaternary ammonium salt, or a protonated quaternary phosphonium salt. As shown by Reedy et al, U.S. Pat. No. 3,983,148, the cationic surface agent, which is essentially soluble only in the aqueous phase, is selected from a limited class of salts of protonated compounds. In addition, Reedy et al's catalysts are subject to acid catalyzed elimination reactions which form products capable of dissolving in and contaminating the resulting cyclopolydimethylsiloxanes.
In my copending application Ser. No. 408,103, cyclic polydimethylsiloxane is described having the formula, EQU [(CH.sub.3).sub.2 SiO].sub.m ( 1)
where m is a whole number equal to from 3-6 inclusive. These cyclics can be made by hydrolyzing dimethyldichlorosilane in the presence of an n-C.sub.(6-16) alkyl sulfonic acid. The amount of alkyl sulfonic acid which can be utilized as a hydrolysis catalyst can vary from about 0.1% to about 2% by weight based on the weight of aqueous HCl hydrolysis medium. In instances where the tetramer is desired, where n formula (1) is equal to 4, it has been found that from about 0.25% to 2% by weight, based on the weight of aqueous HCl of alkyl sulfonic acid catalyst will provide effective results.
The present invention is based on my discovery that further improvements in making cyclopolydimethylsiloxanes of formula (1) can be realized if a perfluoroalkylsulfonic acid catalyst is used in place of the aforementioned alkylsulfonic acid. I have found for example, that perfluoroalkylsulfonic acid salts having the formula, EQU (C.sub.x F.sub.2x+1)SO.sub.3 M (2)
where M is an alkali metal such as potassium, sodium, lithium, or a tetralkylammonium radical, and x is an integer having a value of 6 to 16, can be utilized to make cyclic polydimethylsiloxane of formula (1) at concentrations below 0.1% weight based on the weight of the aqueous HCl hydrolysis medium. Preferably, the perfluoroalkylsulfonic acid of formula (2) can be used at 200 ppm to about 600 ppm based on HCL hydrolysis medium weight. Surprisingly, octamethylcyclotetrasiloxane or "tetramer" can be made in yields as high as 79% by weight based on the weight of the product mixture when utilizing the perfluoroalkylsulfonic acid catalyst at 400 ppm based on the weight of the aqueous HCl hydrolysis medium which can be at concentrations as high as 36% HCl by weight.